1. Field of the Invention
The present invention relates, in general, to a catalyst for oxidizing hydrogen sulfide gas to elemental sulfur and, more particularly, to a catalyst capable of selectively oxidizing hydrogen sulfide gas even when it contains moisture. Also, the present invention relates to a method for recovering elemental sulfur from hydrogen sulfide gas using the catalyst.
2. Description of the Prior Art
Representative of air pollutants, hydrogen sulfide is a colorless, toxic gas, giving out a bad smell. Hydrogen sulfide is produced in large quantity as a main or side product of biological metabolisms or industrial processes. Once hydrogen sulfide, whose sources are various at present, is released to the air, living organisms, including humans, absorb or adsorb the toxic gas directly or indirectly. In this course, the living organisms may suffer a fatal blow owing to the serious toxicity of hydrogen sulfide. Further, industrial facilities are increasingly deteriorated when coming into contact with hydrogen sulfide.
Since it is virtually impossible to effectively remove hydrogen sulfide from hydrogen sulfide-contaminated air, the best policy is to reduce the amount of the hydrogen sulfide gas released to the air. In this regard, there is a tendency toward the strengthening of laws regulating the hydrogen sulfide discharging industries. However, regulation by law is not preferable because the hydrogen sulfide discharging industries make a contribution to national economy. Rather than controlling hydrogen sulfide sources, appropriately processing inevitably generated hydrogen sulfide in advance of the discharge of hydrogen sulfide to the air is desired. That is, preferable is that an appropriate desulfurization process is adopted at the end of various processes in industrial facilities.
When fossil fuels themselves are directly processed or used as energy sources, a large quantity of hydrogen sulfide is produced. Relevant industrial facilities are representatively exemplified by oil refineries, iron manufacturing plants, and power plants. The effects of the quantity of the hydrogen sulfide generated from such large-scale plant facilities are not limited to local contamination, but may directly pollute other nations or the entire globe. Thus, the removal of hydrogen sulfide is of particular concern to all nations at present.
Claus reaction for converting toxic hydrogen sulfide into elemental sulfur, which is non-toxic to the body and the environment is one of the most well-known hydrogen sulfide removing methods. In a hydrogen sulfide-removing method using Claus reaction, toxic hydrogen sulfide is oxidized to elemental sulfur by passing through a reaction system comprising one high temperature furnace and two or three catalyst reactors, where thermal oxidation and catalytic reaction occur.
In detail, in the thermal oxidation process, one-third of the fed hydrogen sulfide(H2S) is oxidized to sulfur dioxide(SO2) in the high temperature furnace maintained at 1,100-1,200xc2x0 C. This thermal oxidation is explained by the following reaction formula 1:
2H2S+3O2xe2x86x922SO2+2H2O
In the catalyst reaction process, which is subsequent to the thermal oxidation process, unconverted hydrogen sulfide and the sulfur dioxide produced in the high temperature furnace are mixed in a molar ratio of 2:1 and converted elemental sulfur through the condensation reaction, named Claus reaction, represented by the following reaction formula 2:
2H2S+SO2←xe2x86x923/n Sn+2H2O
The Claus process including the condensation of the reaction formula 2 suffers limited sulfur-recovery efficiency for the following reasons, making it difficult to increase hydrogen sulfide treating efficiency to a desirable level.
First, because the Claus process according to the reaction formula 2 is reversible chemically and thermodynamically, the equilibrium conversion rate is limited.
Next, the forward reaction, as shown in the reaction formula 2, is smoothly progressed with maintenance of the stoichiometric molar ratio of hydrogen sulfide to sulfide dioxide at 2:1. However, the stoichiometric molar ratio is not easy to control into the quantitatively accurate value. Thus, the forward reaction rate is lowered.
Finally, it is somewhat difficult to remove the water produced by the forward reaction, so the reverse reaction of the reaction formula 2 may predominate over the forward reaction. Accordingly, there may be caused a result contrary to the desulfurization purpose of converting hydrogen sulfide to elemental sulfur.
When the Claus process is used to treat a volume of hydrogen sulfide, 3-5% of the volume of the hydrogen sulfide typically remains unreacted owing to the above-mentioned problems. Usually, such tail gas is incinerated for discharge to the air. Incineration of hydrogen sulfide leads to sulfur dioxide as a main product. This incineration has been blamed for the emission of the serious air pollutant sulfur dioxide.
To avoid the pollution of the air, the tail gas must be further treated. In fact, a number of Claus tail-gas treatment processes have been developed to increase the total sulfur-recovery efficiency. Most of the conventional Claus tail-gas treatment processes, which take advantage of the adsorption or absorption of hydrogen sulfide, however, have the disadvantage of inhibiting the continuous operation of the facilities because of producing wastes after the treatment or requiring separate, periodic recycle processes.
In the most efficient Claus tail-gas treatment process, the removal of toxic hydrogen sulfide utilizes a catalyst. For example, a tail gas containing sulfur is hydrogenated to give hydrogen sulfide which is then oxidized on a catalyst to elemental sulfur. The following reaction formula 3 explains this reaction:
2H2S+O2xe2x86x922/n Sn+2H2O
The Claus tail-gas treatment processes following the reaction formula 3 are representative of a mobile direct oxidation process (MODOP), which is high in sulfur-recovery rate as disclosed in EP 0 078 690 A2, and a super-Claus process which is disclosed in U.S. Pat. No. 5,286,697.
In the MODOP, hydrogen sulfide is directly converted to elemental sulfur by being reacted with oxygen at the stoichiometric molar ratio (2:1) in the presence of a titanium dioxide (TiO2)-based catalyst. Through the three-step process including the Claus process, the conversion of hydrogen sulfide to elemental sulfur is achieved at a rate of as high as 90% or higher. The MODOP requires that the moisture level of the reaction gas be reduced to less than 4% prior to the catalytic reaction as the catalyst may be functionally deteriorated by water poisoning. Thus, the MODOP has the prerequisite condition of conducting a dehydration process in advance of the catalytic process, causing complexity in the conversion procedure.
The super Claus process is similar to the MODOP, but more useful in terms of requiring no separate dehydration processes. In other words, the iron- or chrome-based catalyst used in the super Claus process is not seriously vulnerable to moisture. Therefore, the super Claus process allows hydrogen sulfide gas containing excess moisture to be directly converted to elemental sulfur, showing as high a sulfur-recovery efficiency as that of the MODOP. Instead of the dehydration as in MODOP process, however, a prerequisite process is needed to prevent the catalyst from being poisoned by water and limiting the reverse super Claus reaction. The activity of the catalyst cannot be maintained high enough to drive the super Claus reaction without excessively using oxygen at an amount ten-fold larger than the stoichiometric equivalent required in the reaction formula 3. Indeed, the super Claus chemical reaction is smoothly conducted when the hydrogen sulfide gas is maintained at the level of less than 1 vol %. If the amount of the hydrogen sulfide gas exceeds 2 vol %, the hydrogen sulfide is difficult to treat by the super Claus process.
Therefore, there remains a need for a catalyst that requires only the stoichiometric amount of hydrogen sulfide to drive the chemical reaction without being poisoned even in the presence of excess water (moisture). Thus far, there have not developed the catalysts which meet the above requirements.
In many patents, catalysts that show activity in the substantial absence of moisture or in the presence of less than 5% of moisture are disclosed.
For example, U.S. Pat. No. 4,311,683 describes a V2O5 catalyst supported on a non-basic carrier, with which the sulfur-recovery rate can be increased to 75-90% by reacting an acidic gas containing 500 ppm or 10 vol % of hydrogen sulfide with oxygen at the stoichiometric equivalent ratio at 232xc2x0 C. under a pressure of 100 psig.
U.S. Pat. Nos. 4,444,743 and 4,576,814 disclose the two-component catalyst V2O5xe2x80x94Bi2O3 in the presence of which the sulfur recovery can be achieved at a rate of 70-80% at 246xc2x0 C. or lower in an atmosphere containing about 3 vol % of moisture.
VMoOx and VMgx/SiO2 catalyst systems are disclosed in U.S. Pat. No. 5,653,953, a Fe2O3-based catalyst system containing Ce, Tn or Sb in U.S. Pat. No. 5,700,440 and a Bi2O3-based catalyst system containing Mo and V in U.S. Pat. No. 5,597,546. These catalyst systems are reported to afford a maximum sulfur-recovery rate of 95% when as low as 0.8-3 vol % of hydrogen sulfide is reacted with ten-fold larger equivalents than the stoichiometric amount of oxygen at 200-280xc2x0 C.
U.S. Pat. Nos. 5,603,913 and 5,891,415 disclose an FeAMgBZnCCrDOx catalyst (wherein, 0.5xe2x89xa6Axe2x89xa610, 0.1xe2x89xa6Bxe2x89xa61, 0xe2x89xa6Cxe2x89xa61, 0xe2x89xa6Dxe2x89xa61, B+C=1) and an FeAZnBOX catalyst (wherein 0.5xe2x89xa6Axe2x89xa610, 1xe2x89xa6Bxe2x89xa62), respectively. Both the two catalysts are reported to guarantee a sulfur-recovery rate of 93% when hydrogen sulfide is reacted with oxygen in the stoichiometric equivalent ratio in the substantial absence of water.
An excess moisture-allowable reaction condition can be found in U.S. Pat. No. 5,512,258 which discloses an A4xc2x1xV2xc2x1yO9 catalyst system (wherein A is selected from the group consisting of Mg, Ca and Zn, 0xe2x89xa6xxe2x89xa60.2 , 0xe2x89xa6yxe2x89xa60.5). Reported is that a mix gas comprising 2 vol % of hydrogen sulfide, 1 vol % of sulfur dioxide and an appropriate stoichiometric amount (about 1 vol %) of oxygen is treated with a reaction gas containing 30 vol % of moisture at 220xc2x0 C., the conversion rate of hydrogen sulfide can be increased to as high as 70% and 90% at the maximum. However, nowhere is mentioned the selectivity for sulfur. In addition, the sulfur-recovery rate does not reach a desired level.
Most of the conventional techniques, as mentioned above, show only the results which are obtained by conducting processes under the conditions containing no moisture or small amounts of moisture. When the catalysts in prior arts are used in practical conditions, which contain a large volume of moisture, the results promised by the patents cannot be guaranteed. Rather, when moisture is present in the sulfur produced at high temperatures, reverse super Claus reaction occurs owing to the alkalinity of the catalyst, producing sulfur dioxide as a by-product and lowering the sulfur selectivity. To circumvent the deterioration of sulfur selectivity even to a small degree, the conversion is conventionally conducted in a low temperature condition, for example, at 245xc2x0 C. or lower. U.S. Pat. No. 4,311,683 and 5,512,258, however, teaches that, while working for a long period of time at such a low temperature, the catalyst V2O5 is converted to a sulfide such as VO(SO4) or VSx, losing its catalytic activity gradually and finally to the dead point.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide a catalyst for the selective oxidation of hydrogen sulfide, which can maintain a high sulfur-recovery rate even in the presence of excess moisture in the reaction gas at a low temperature condition of less than 245xc2x0 C. as well as at a high temperature condition of 245-330xc2x0 C. and exhibits no inactivation according to working time.
It is another object of the present invention to provide a method for recovering elemental sulfur by the selective oxidation of hydrogen sulfide, which is very effectively conducted in the presence of the catalyst even in a condition containing excess moisture.
In one embodiment of the present invention, there is provided a catalyst for selectively oxidizing hydrogen sulfide to elemental sulfur, represented by the following chemical formula 1:
VaTibXcOfxe2x80x83xe2x80x831
wherein, a is such a mole number that vanadium amounts to 5-40% by weight based on the total weight of the catalyst; b is such a mole number that titanium amounts to 5-40% by weight based on the total weight of the catalyst; X is an element selected from the group consisting of Fe, Mn, Co, Ni, Sb and Bi; c is such a mole number that X amounts to 15% by weight or less based on the total weight of the catalyst; and f is such a mole number that oxygen is present to the final 100% by weight.
In one version of the embodiment, the catalyst further contains an element component Y selected from the group consisting of Cr, Mo, W, Zr, Zn, Sn and mixtures thereof, represented by the following chemical formula 2:
VaTibXcYdOfxe2x80x83xe2x80x832
wherein,
a, b, c, f and X are each as defined above; and
d is such a mole number that the element component Y amounts to 8% by weight or less based on the total weight of the catalyst.
In another version of the embodiment, the catalyst further contains an element component Z selected from the group consisting of Mg, Ca, Sr, Cs, La, Ce and mixtures thereof, represented by the following chemical formula, 3:
VaTibXcYdZeOfxe2x80x83xe2x80x833
wherein,
a, b, c, f, X and Y are each as defined above; and
e is such a mole number that the element component Z amounts to 8% by weight or less based on the total weight of the catalyst.
In a further version of the embodiment, the catalyst further contains element component Z selected from the groups consisting of Mg, Ca, Sr, Cs, La, Ce and mixtures thereof, represented by the following chemical formula 4:
VaTibXcZeOfxe2x80x83xe2x80x834
wherein,
a, b, c, f and X are each as defined above; and
e is such a mole number that the element component Z amounts to 8% by weight or less based on the total weight of the catalyst.
In another embodiment of the present invention, there is provided a catalyst for selectively oxidizing hydrogen sulfide to elemental sulfur, represented by the following chemical formula 5:
VaTibYdOfxe2x80x83xe2x80x835
wherein,
a is such a mole number that vanadium amounts to 5-40% by weight based on the total weight of the catalyst;
b is such a mole number that titanium amounts to 5-40% by weight based on the total weight of the catalyst;
Y is an element selected from the group consisting of Mo, Zr, Zn, Ce, Sn and mixtures thereof;
d is such a mole number that Y amounts to 8% by weight or less based on the total weight of the catalyst; and
f is such a mole number that oxygen is contained to the final 100% by weight.
In one version of this embodiment, the catalyst further contains an element component Z selected from the groups consisting of Mg, Ca, Sr, Cs, La, Ce and mixtures thereof, represented by the following chemical formula 6:
VaTibYdZeOfxe2x80x83xe2x80x836
wherein,
a, b, d, f and Y are each as defined above; and
e is such a mole number that the element component Z amounts to 8% by weight or less based on the total weight of the catalyst.
The catalysts may be metal composite oxides. Alternatively, the catalysts may be dispersed and supported on non-acidic or non-alkaline carrier particles. This is preferred because the catalytically active particles can be reduced in amount with an increase in surface area. As for the carrier, it is selected from the group consisting of titanium dioxide, silicon carbide and silicon dioxide. In the case of using titanium dioxide as a carrier, the Ti component contained in the metal composite oxide may be omitted. When being supported on a carrier, the catalytically active metal composite oxide preferably amounts to 5-50% by weight based on the total weight of the catalyst comprising the carrier.
In accordance with a further embodiment of the present invention, there is provided a method for recovering elemental sulfur by selectively oxidizing a reaction gas containing 0.5-40 vol % of hydrogen sulfide with oxygen in the presence of the catalyst.
In the method of the present invention, reaction gas containing H2S preferably contains moisture at an amount of 50 vol % or less based on the total volume of a reaction gas containing H2S. The reaction between the reaction gas and oxygen is conducted at 200-350xc2x0 C. with a volume ratio of oxygen to hydrogen sulfide ranging from 0.5 to 1. The reaction gas is fed at a space velocity of 3,000-1,000,000/hr. More preferably, the reaction gas is fed at a space velocity of 3,000-400,000/hr and reacted with oxygen at 200-280xc2x0 C. with a volume ratio of oxygen to hydrogen sulfide ranging from 0.5 to 0.6.
The oxidation of hydrogen sulfide on the metal composite oxide results from the catalytic oxidation-reduction mechanism in which the catalyst is reduced by the hydrogen sulfide to lose lattice oxygen and gas phase oxygen donates lattice oxygen to restore the catalytic activity of the catalyst. Therefore, the activation of the oxidation and its maintenance depend on the quantity of lattice oxygen within the vanadium which is the main catalytic element and on the reoxidation of the catalyst by gas phase oxygen, which is facilitated by the accompanying metal oxides including titanium dioxide. Accordingly, if the amount of any one of the catalytically active components is too small or if the difference between the amount of two different catalytically active components is large, a disruption is brought about in the balance between the reducing power of H2S and the oxidizing power of oxygen within the catalyst, resulting in inactivating the catalyst. Particularly at low temperatures, the activity of the catalyst may be rapidly decreased with time owing to poor reoxidation. Conventional vanadium-based catalysts inevitably experience such inactivation. In contrast, the catalyst of the present invention is not inactivated because the reoxidation of the catalyst by oxygen is facilitated by composite elements of the metal composite oxide other than vanadium, including titanium. In addition, the catalyst and method of the present invention can recover sulfur with high efficiency even in the condition containing a large volume of moisture.
From vanadium oxide, titanium dioxide and the other metal oxides, the catalyst is prepared into a mixed oxide or composite oxide form. Any preparation method may be used. Available as a precursor for the vanadium in the catalyst is NH4VO3, VOCl3, VOSO4.3H2O, or vanadium alkoxides. For providing the titanium component of the catalyst, Ti(SO4)2.nH2l O, TiCl3 or various titanium alkoxides may be used as a precursor. The titanium precursors may be of various phases. Any titanium precursors may be used if they are soluble in aqueous acid solutions, alcohol or other solutions.
By way of example, but not limitation, NH4VO3, Ti[O(CH2)3CH3]4, Fe(NO3)3.9H2O, or (NH4)6Mo7O2 or Cr(NO3)3.9H2O is dissolved in an aqueous oxalic acid solution or nitric acid solution, followed by the removal of moisture by evaporation or precipitation to give a precursor solid of a multi-component metal composite containing vanadium and titanium, such as VaTibFecModCreOf. (wherein, a,b,c,d,e and f are each as defined above.) Any known metal salt can be used as a precursor for the metal element of X, Y and Z.
Alternatively, a slurry of carrier particles such as solid silica is added to the solution which is then dried by evaporation to prepare an active catalyst precursor supported on the silica carrier.
An alkaline solution such an aqueous NH4OH solution is also useful for the preparation of the catalyst. Introduction of the alkaline solution to the reaction solution gives precipitates which can be used as the catalyst after being filtered off. In another preparation method, an aqueous NH4VO3 solution is added with a slurry of titanium dioxide, followed by evaporation drying or precipitating to give final precursor solids.
The multi-component metal composite oxide precursor solid obtained by any of the preparation method is sintered at 400-500xc2x0 C. in the air or in an oxygen atmosphere to produce a multi-component metal composite oxide catalyst containing vanadium and titanium.
Catalysts having chemical formula, VaTibFecModOf and VaTibFecModCreOf(wherein, a,b,c,d,e and f are each as defined the above) are preferable.
Preferably, the amount of vanadium in the catalysts is 10 to 50 wt% of titanium and weight ratio of amount of each metal element for X, Y and Z to vanadium is 1:5 to 1:3.
In the presence of the catalyst, a hydrogen sulfide-containing gas is reacted with oxygen or air at 200-350xc2x0 C. to oxidize hydrogen sulfide to elemental sulfur. The control of the reaction temperature is very important to increase the total sulfur-recovery efficiency. For example, if the reaction temperature is lowered to less than 200xc2x0 C., the catalyst is inactivated. On the other hand, if the reaction temperature exceeds 350xc2x0 C., the sulfur yield is decreased.